add stubs for dlopen, dlsym, etc. (#443)

* add stubs for dlopen, dlsym, etc.

This adds weak exports for the POSIX `dlopen`, `dlsym`, `dlclose`, and `dlerror`
functions, allowing code which uses those features to compile.  The
implementations are stubs which always fail since there is currently no official
standard for runtime dynamic linking.

Since the symbols are weak, they can be overriden with useful, runtime-specific
implementations, e.g. based on host functions or statically-generated tables
(see https://github.com/dicej/component-linking-demo for an example of the
latter).

Signed-off-by: Joel Dice <joel.dice@fermyon.com>

* move `dlopen` stubs out of libc and into libdl

Per review feedback, it's easier to simply replace libdl.so with a working
implementation at runtime than it is to override a handful of symbols in libc.

Note that I've both added libdl.so and replaced the empty libdl.a we were
previously creating with one that contains the stubs.  I'm thinking we might as
well be consistent about what symbols the .so and the .a contain.  Otherwise,
e.g. the CPython build gets confused when the dlfcn.h says `dlopen` etc. exist
but libdl.a is empty.

Signed-off-by: Joel Dice <joel.dice@fermyon.com>

* customize dlfcn.h for WASI

For WASI, we use flag values which match MacOS rather than musl.  This gives
`RTLD_LOCAL` a non-zero value, avoiding ambiguity and allowing us to defer the
decision of whether `RTLD_LOCAL` or `RTLD_GLOBAL` should be the default when
neither is specified.

We also avoid declaring `dladdr`, `dlinfo`, and friends on WASI since they are
neither supported nor stubbed at this time.

Signed-off-by: Joel Dice <joel.dice@fermyon.com>

* use musl's RTLD_* flags except for RTLD_LOCAL

This minimizes the divergence from upstream while still giving us the
flexibility to choose a default value later.

Signed-off-by: Joel Dice <joel.dice@fermyon.com>

* use `NULL` instead of `0` for null pointers

Signed-off-by: Joel Dice <joel.dice@fermyon.com>

---------

Signed-off-by: Joel Dice <joel.dice@fermyon.com>

Author: dicej

Makefile

@@ -80,6 +80,7 @@ LIBWASI_EMULATED_SIGNAL_SOURCES = \
 LIBWASI_EMULATED_SIGNAL_MUSL_SOURCES = \
     $(LIBC_TOP_HALF_MUSL_SRC_DIR)/signal/psignal.c \
     $(LIBC_TOP_HALF_MUSL_SRC_DIR)/string/strsignal.c
+LIBDL_SOURCES = $(LIBC_TOP_HALF_MUSL_SRC_DIR)/misc/dl.c
 LIBC_BOTTOM_HALF_CRT_SOURCES = $(wildcard $(LIBC_BOTTOM_HALF_DIR)/crt/*.c)
 LIBC_TOP_HALF_DIR = libc-top-half
 LIBC_TOP_HALF_MUSL_DIR = $(LIBC_TOP_HALF_DIR)/musl
@@ -379,6 +380,7 @@ LIBWASI_EMULATED_PROCESS_CLOCKS_OBJS = $(call objs,$(LIBWASI_EMULATED_PROCESS_CL
 LIBWASI_EMULATED_GETPID_OBJS = $(call objs,$(LIBWASI_EMULATED_GETPID_SOURCES))
 LIBWASI_EMULATED_SIGNAL_OBJS = $(call objs,$(LIBWASI_EMULATED_SIGNAL_SOURCES))
 LIBWASI_EMULATED_SIGNAL_MUSL_OBJS = $(call objs,$(LIBWASI_EMULATED_SIGNAL_MUSL_SOURCES))
+LIBDL_OBJS = $(call objs,$(LIBDL_SOURCES))
 LIBC_BOTTOM_HALF_CRT_OBJS = $(call objs,$(LIBC_BOTTOM_HALF_CRT_SOURCES))
 
 # These variables describe the locations of various files and
@@ -445,7 +447,6 @@ MUSL_OMIT_HEADERS += \
     "netdb.h" \
     "resolv.h" \
     "pty.h" \
-    "dlfcn.h" \
     "setjmp.h" \
     "ulimit.h" \
     "sys/xattr.h" \
@@ -481,6 +482,7 @@ LIBWASI_EMULATED_PROCESS_CLOCKS_SO_OBJS = $(patsubst %.o,%.pic.o,$(LIBWASI_EMULA
 LIBWASI_EMULATED_GETPID_SO_OBJS = $(patsubst %.o,%.pic.o,$(LIBWASI_EMULATED_GETPID_OBJS))
 LIBWASI_EMULATED_SIGNAL_SO_OBJS = $(patsubst %.o,%.pic.o,$(LIBWASI_EMULATED_SIGNAL_OBJS))
 LIBWASI_EMULATED_SIGNAL_MUSL_SO_OBJS = $(patsubst %.o,%.pic.o,$(LIBWASI_EMULATED_SIGNAL_MUSL_OBJS))
+LIBDL_SO_OBJS = $(patsubst %.o,%.pic.o,$(LIBDL_OBJS))
 BULK_MEMORY_SO_OBJS = $(patsubst %.o,%.pic.o,$(BULK_MEMORY_OBJS))
 DLMALLOC_SO_OBJS = $(patsubst %.o,%.pic.o,$(DLMALLOC_OBJS))
 LIBC_BOTTOM_HALF_ALL_SO_OBJS = $(patsubst %.o,%.pic.o,$(LIBC_BOTTOM_HALF_ALL_OBJS))
@@ -521,6 +523,8 @@ $(OBJDIR)/libwasi-emulated-getpid.so.a: $(LIBWASI_EMULATED_GETPID_SO_OBJS)
 
 $(OBJDIR)/libwasi-emulated-signal.so.a: $(LIBWASI_EMULATED_SIGNAL_SO_OBJS) $(LIBWASI_EMULATED_SIGNAL_MUSL_SO_OBJS)
 
+$(OBJDIR)/libdl.so.a: $(LIBDL_SO_OBJS)
+
 $(SYSROOT_LIB)/libc.a: $(LIBC_OBJS)
 
 $(SYSROOT_LIB)/libc-printscan-long-double.a: $(MUSL_PRINTSCAN_LONG_DOUBLE_OBJS)
@@ -535,6 +539,8 @@ $(SYSROOT_LIB)/libwasi-emulated-getpid.a: $(LIBWASI_EMULATED_GETPID_OBJS)
 
 $(SYSROOT_LIB)/libwasi-emulated-signal.a: $(LIBWASI_EMULATED_SIGNAL_OBJS) $(LIBWASI_EMULATED_SIGNAL_MUSL_OBJS)
 
+$(SYSROOT_LIB)/libdl.a: $(LIBDL_OBJS)
+
 %.a:
 	@mkdir -p "$(@D)"
 	# On Windows, the commandline for the ar invocation got too long, so it needs to be split up.
@@ -603,7 +609,7 @@ startup_files $(LIBC_BOTTOM_HALF_ALL_OBJS) $(LIBC_BOTTOM_HALF_ALL_SO_OBJS): CFLA
     -I$(LIBC_TOP_HALF_MUSL_SRC_DIR)/include \
     -I$(LIBC_TOP_HALF_MUSL_SRC_DIR)/internal
 
-$(LIBC_TOP_HALF_ALL_OBJS) $(LIBC_TOP_HALF_ALL_SO_OBJS) $(MUSL_PRINTSCAN_LONG_DOUBLE_OBJS) $(MUSL_PRINTSCAN_LONG_DOUBLE_SO_OBJS) $(MUSL_PRINTSCAN_NO_FLOATING_POINT_OBJS) $(LIBWASI_EMULATED_SIGNAL_MUSL_OBJS) $(LIBWASI_EMULATED_SIGNAL_MUSL_SO_OBJS): CFLAGS += \
+$(LIBC_TOP_HALF_ALL_OBJS) $(LIBC_TOP_HALF_ALL_SO_OBJS) $(MUSL_PRINTSCAN_LONG_DOUBLE_OBJS) $(MUSL_PRINTSCAN_LONG_DOUBLE_SO_OBJS) $(MUSL_PRINTSCAN_NO_FLOATING_POINT_OBJS) $(LIBWASI_EMULATED_SIGNAL_MUSL_OBJS) $(LIBWASI_EMULATED_SIGNAL_MUSL_SO_OBJS) $(LIBDL_OBJS) $(LIBDL_SO_OBJS): CFLAGS += \
     -I$(LIBC_TOP_HALF_MUSL_SRC_DIR)/include \
     -I$(LIBC_TOP_HALF_MUSL_SRC_DIR)/internal \
     -I$(LIBC_TOP_HALF_MUSL_DIR)/arch/wasm32 \
@@ -664,7 +670,8 @@ LIBC_SO = \
 	$(SYSROOT_LIB)/libwasi-emulated-mman.so \
 	$(SYSROOT_LIB)/libwasi-emulated-process-clocks.so \
 	$(SYSROOT_LIB)/libwasi-emulated-getpid.so \
-	$(SYSROOT_LIB)/libwasi-emulated-signal.so
+	$(SYSROOT_LIB)/libwasi-emulated-signal.so \
+	$(SYSROOT_LIB)/libdl.so
 endif
 
 libc_so: include_dirs $(LIBC_SO)
@@ -676,13 +683,14 @@ libc: include_dirs \
     $(SYSROOT_LIB)/libwasi-emulated-mman.a \
     $(SYSROOT_LIB)/libwasi-emulated-process-clocks.a \
     $(SYSROOT_LIB)/libwasi-emulated-getpid.a \
-    $(SYSROOT_LIB)/libwasi-emulated-signal.a
+    $(SYSROOT_LIB)/libwasi-emulated-signal.a \
+    $(SYSROOT_LIB)/libdl.a
 
 finish: startup_files libc
 	#
 	# Create empty placeholder libraries.
 	#
-	for name in m rt pthread crypt util xnet resolv dl; do \
+	for name in m rt pthread crypt util xnet resolv; do \
 	    $(AR) crs "$(SYSROOT_LIB)/lib$${name}.a"; \
 	done
 

expected/wasm32-wasi-threads/include-all.c

@@ -75,6 +75,7 @@
 #include <crypt.h>
 #include <ctype.h>
 #include <dirent.h>
+#include <dlfcn.h>
 #include <endian.h>
 #include <err.h>
 #include <errno.h>

expected/wasm32-wasi-threads/predefined-macros.txt

@@ -1499,6 +1499,14 @@
 #define RRFIXEDSZ NS_RRFIXEDSZ
 #define RRQ 01
 #define RS_HIPRI 0x01
+#define RTLD_DEFAULT ((void *)0)
+#define RTLD_GLOBAL 256
+#define RTLD_LAZY 1
+#define RTLD_LOCAL 8
+#define RTLD_NEXT ((void *)-1)
+#define RTLD_NODELETE 4096
+#define RTLD_NOLOAD 4
+#define RTLD_NOW 2
 #define RUSAGE_CHILDREN 2
 #define RUSAGE_SELF 1
 #define R_OK (4)
@@ -2044,6 +2052,7 @@
 #define _Complex_I (0.0f+1.0fi)
 #define _DIRENT_H 
 #define _DIRENT_HAVE_D_TYPE 
+#define _DLFCN_H
 #define _ENDIAN_H 
 #define _ERRNO_H 
 #define _ERR_H 

expected/wasm32-wasi/include-all.c

@@ -75,6 +75,7 @@
 #include <crypt.h>
 #include <ctype.h>
 #include <dirent.h>
+#include <dlfcn.h>
 #include <endian.h>
 #include <err.h>
 #include <errno.h>

expected/wasm32-wasi/predefined-macros.txt

@@ -1467,6 +1467,14 @@
 #define RRFIXEDSZ NS_RRFIXEDSZ
 #define RRQ 01
 #define RS_HIPRI 0x01
+#define RTLD_DEFAULT ((void *)0)
+#define RTLD_GLOBAL 256
+#define RTLD_LAZY 1
+#define RTLD_LOCAL 8
+#define RTLD_NEXT ((void *)-1)
+#define RTLD_NODELETE 4096
+#define RTLD_NOLOAD 4
+#define RTLD_NOW 2
 #define RUSAGE_CHILDREN 2
 #define RUSAGE_SELF 1
 #define R_OK (4)
@@ -2010,6 +2018,7 @@
 #define _Complex_I (0.0f+1.0fi)
 #define _DIRENT_H 
 #define _DIRENT_HAVE_D_TYPE 
+#define _DLFCN_H
 #define _ENDIAN_H 
 #define _ERRNO_H 
 #define _ERR_H 

libc-top-half/musl/include/dlfcn.h

@@ -12,19 +12,29 @@ extern "C" {
 #define RTLD_NOLOAD 4
 #define RTLD_NODELETE 4096
 #define RTLD_GLOBAL 256
+#ifdef __wasilibc_unmodified_upstream
 #define RTLD_LOCAL  0
+#else
+/* For WASI, we give `RTLD_LOCAL` a non-zero value, avoiding ambiguity and
+ * allowing us to defer the decision of whether `RTLD_LOCAL` or `RTLD_GLOBAL`
+ * should be the default when neither is specified.
+ */
+#define RTLD_LOCAL  8
+#endif
 
 #define RTLD_NEXT    ((void *)-1)
 #define RTLD_DEFAULT ((void *)0)
 
+#ifdef __wasilibc_unmodified_upstream
 #define RTLD_DI_LINKMAP 2
+#endif
 
 int    dlclose(void *);
 char  *dlerror(void);
 void  *dlopen(const char *, int);
 void  *dlsym(void *__restrict, const char *__restrict);
 
-#if defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
+#if defined(__wasilibc_unmodified_upstream) && (defined(_GNU_SOURCE) || defined(_BSD_SOURCE))
 typedef struct {
 	const char *dli_fname;
 	void *dli_fbase;

libc-top-half/musl/src/misc/dl.c

@@ -0,0 +1,45 @@
+/* This file is used to build libdl.so with stub versions of `dlopen`, `dlsym`,
+ * etc.  The intention is that this stubbed libdl.so can be used to build
+ * libraries and applications which use `dlopen` without committing to a
+ * specific runtime implementation.  Later, it can be replaced with a real,
+ * working libdl.so (e.g. at runtime or component composition time).
+ * 
+ * For example, the `wasm-tools component link` subcommand can be used to create
+ * a component that bundles any `dlopen`-able libraries in such a way that their
+ * function exports can be resolved symbolically at runtime using an
+ * implementation of libdl.so designed for that purpose.  In other cases, a
+ * runtime might provide Emscripten-style dynamic linking via URLs or else a
+ * more traditional, filesystem-based implementation.  Finally, even this
+ * stubbed version of libdl.so can be used at runtime in cases where dynamic
+ * library resolution cannot or should not be supported (and the application can
+ * handle this situation gracefully). */
+
+#include <stddef.h>
+#include <dlfcn.h>
+
+static const char *error = NULL;
+
+weak int dlclose(void *library)
+{
+	error = "dlclose not implemented";
+	return -1;
+}
+
+weak char *dlerror(void)
+{
+	const char *var = error;
+	error = NULL;
+	return (char*) var;
+}
+
+weak void *dlopen(const char *name, int flags)
+{
+	error = "dlopen not implemented";
+	return NULL;
+}
+
+weak void *dlsym(void *library, const char *name)
+{
+	error = "dlsym not implemented";
+	return NULL;
+}